An automatic transmission that incorporates an electromagnetic valve and adjusts variables relating to a gear shift operation, such as a gear, a hydraulic pressure level, a time constant or timing in the gear shift operation, and the like, by means of an ECU (Electronic Control Unit) by receiving an electrical signal from the outside has been put into use. Such an ECU allows reliable and prompt transition of an operation state of the automatic transmission to various states. In addition, as the ECU incorporates a CPU (Central Processing Unit), control using program can be carried out. Accordingly, if an operation state of the automatic transmission is finely set by modifying a program or various constants, optimal performance of the automatic transmission can be obtained in line with a running state of a vehicle or a state of load onto an engine. Here, the running state of the vehicle refers to a vehicle speed, a steering operation, frequency and a level of acceleration and deceleration, a road surface state, and the like, while the state of load onto the engine refers to an engine speed, a throttle position, an accelerator pedal pressing degree, torque of input and output shafts of an engine or an automatic transmission, and the like.
A technique to finely tune a hydraulic pressure level applied to a frictional engagement element (a clutch or a brake) contained in the automatic transmission in accordance with the running state of the vehicle or the state of load onto the engine has been known. With such a technique, suppression of gear shift shock and reduction in wear of a fastening element can both be achieved, and rapid and smooth gear shift can be attained.
Japanese Patent Laying-Open No. 08-303567 discloses a control device of an automatic transmission capable of appropriately and finely setting a hydraulic pressure level or the like with smaller number of data pieces to be handled. The control device of the automatic transmission taking in, from hour to hour, operational information reflecting engine load and determining a control constant for the automatic transmission based on at least one of the operational information, a vehicle speed, and an engine speed, includes: storage means for storing two or more types of maps of control constants determined in correspondence with a drive region where motive power is transmitted in a direction in which the engine drives the automatic transmission and a coast region where the engine is in such an idle state that it cannot drive the automatic transmission; identification means for identifying whether a drive state of the engine is either in the drive region or in the coast region by detecting the operational information and the engine speed; and setting means for finding a control constant by selecting one of the two or more types of maps stored in the storage means in accordance with a result of identification by the identification means.
According to the control device of the automatic transmission, the drive region and the coast region are distinguished from each other based on combination of the operational information (a throttle position and an accelerator pedal pressing degree) and the engine speed. Accordingly, for example, two types of maps of hydraulic pressure setting values determined based on the throttle position and the vehicle speed are only necessary as in such an example as a hydraulic pressure setting value being associated with ON-OFF of an idle switch, and in addition, the coast region can still be identified in a throttle position region where the idle switch is switched OFF.
A state of the vehicle, however, is different, depending on an engagement state under control with flexibility, of a lockup clutch of a torque converter with lockup clutch of the automatic transmission, a gradient of a road surface, a state of a braking operation, a fast idle control state, and the like. This publication does not interpret such a difference in the state of the vehicle as a factor of fluctuation in gear shift characteristics. Namely, as the publication is merely directed to identification of two regions, i.e., the drive region and the coast region, variation in gear shift shock exists.